Decreased extracellular pH is associated with cancer and ischemic diseases such as stroke, ischemic heart disease, and kidney disease. pH could be a very useful biomarker to identify disease and monitor response to therapy, but it remains a challenge to routinely assess pH in vivo. Methods for measuring pH are either invasive or suffer from low sensitivity, e.g. magnetic resonance (MR) spectroscopy. MR imaging probes are detected indirectly via their interaction with water molecules and the MR signal depends on both the concentration of probe and a molecular property of the probe termed relaxivity. For certain MR probes, relaxivity can change if pH changes. In principle, such responsive probes could act as non-invasive pH sensors. However since the MR signal change in tissue depends on both relaxivity and probe concentration (two unknowns), this severely limits the utility of such a responsive probe. On the other hand, positron emission tomography (PET) can quantitatively estimate probe concentration. We hypothesize that incorporation of a PET isotope into a responsive MR probe and using simultaneous PET-MR imaging will enable determination of both probe concentration and relaxivity and thereby provide a quantitative map of pH. We will incorporate a positron emitting fluorine-18 label into the established pH-responsive MR probe GdDOTA-4AMP. Using a new combined PET-MRI device, we will simultaneously measure the MR and PET signals in a model of muscle ischemia that results in decreased pH. The PET data will be modeled to determine tissue concentration of the probe as function of time. This concentration data will be used to extract the time- and spatially dependent relaxivity data from the MR signal and enable us to generate quantitative pH maps. Using PET-MR to identify tissue with low pH may impact patient diagnosis and provide a tool to monitor how the patient responds to treatment. Besides pH, MR probes can be made to be responsive to other environmental factors like temperature, enzymatic activity, ion flux, or metabolite concentrations. To date, these responsive MR probes have been limited by the inability to factor MR signal into relaxivity and gadolinium concentration in vivo. A dual PET-MR probe may solve this problem. The in vivo proof of concept studies described here could open up scores of possibilities for noninvasive quantitative imaging of enzyme activity, ion flux, and metabolite concentrations. PUBLIC HEALTH RELEVANCE: Low pH (acidosis) is linked to a number of diseases such as cancer, stroke, and heart disease, but it is very difficult to noninvasively measure in patients. We are developing a new imaging test that will enable the measurement of pH and which may prove useful in diagnosis of disease or for monitoring how patients are responding to therapy.